Tool holders 3 of the prior art for use with this type of machining center with an automatic tool exchanger, as shown in FIG. 10, have a collar 5 integrally formed in a position adjacent to a tapered shank 4, which collar 5 is adapted to be grasped by a manipulator arm for the automatic exchange of tool holders. The tool holder has a substantially V-shaped circumferentially extending groove 5a, and a pair of opposed annular collar projections 5b and 5c each disposed on an opposite side of the groove 5a, one close to the tapered shank 4 and the other on the opposite side of the groove, away from the tapered shank 4, respectively. The tapered shank 4 of the tool holder 3 is adapted to be fitted into and engaged with a flared hole 2 formed in a rotary or non-rotating main spindle (hereinafter referred to as "main spindle") 1 of a machine tool.
The main spindle 1 and the tool holder 3 are of standard dimensions, such as the maximum diameter D1 and length L of the flared hole 2 and the tapered shank 4, the external diameter D2 of the main spindle 2, and the external diameter D3 of the collar 5 according to the Japanese Industrial Standards (JIS), and the ISO's International Standards. JIS and the International Standards require that when the tapered shank 4 of a standard tool holder 3 is fitted into the flared hole 2 of a standard main spindle 1, there shall be a predetermined clearance Y between a reference end face 1a of the main spindle 1 and an opposing end face 5d of the collar 5 of the tool holder 3 in anticipation of a certain extent of tolerance deviation in the production of the main spindle and the tool holder.
Such an arrangement can bring the tapered shank securely 4 into tight contact with the flared hole 2, even though the reference end face 1a of the main spindle 1 extends slightly toward the end face 5d of the collar 5 by a tolerance deviation in the production of the main spindle 1, or even though the end face 5d of the collar 5 extends slightly toward the reference end face 1a of the main spindle 1 by a tolerance deviation in the production of the tool holder 3, as illustrated by the broken lines in FIG. 10, because the aforesaid clearance Y accommodates the tolerance deviation to prevent the reference end face 1a from coming into contact with the end face 5d of the collar.
For example, according to JIS Standard No. BT50, and ISO's International Standard No. IT50, the maximum diameters D1 and length L of the flared hole 2 and tapered shank 4 shall be 69,850 mm and 101.08 mm, respectively, the external diameter D2 of the main spindle 1 is to be 128.570 mm, the external diameter D3 of the collar 5 is to be 100 mm, and the deviation accommodating clearance Y between the reference end face 1a of the main spindle 1 and the opposing end face 5d of the tool holder 3 is to be 3 mm according to the JIS Standard, and 3.20 mm per the ISO International Standard.
However, the aforesaid arrangement is substantially disadvantageous in that the flared hole 2 and the tapered shank 4 are likely to undergo substantial friction between the respective contact surfaces due to a fretting, corrosion, or the like wear, since the spatial separation between the reference end face 1a of the main spindle 1 and the collar end face 5d of the tool holder 3 with the aforesaid clearance Y causes a machining load to concentrate on the tapered shank.
In view of this disadvantage, it has been proposed that the fixed reference end face 1a of the main spindle 1 be extended toward end face 5d of the collar of the tool holder 3, as shown in FIG. 11, by precision processing, by a width of the prescribed clearance Y now to an extended end face 1a'. Alternatively, the end face 5d of the fixed collar of the tool holder 3 is extended toward the reference end face 1a, as shown in FIG. 12, by precision processing by a width of the prescribed clearance Y now 5d'. Thus the tapered shank 4 will fit tightly into the flared hole 2 and also bring the aforesaid extended end face 1a' of the collar and the end face 5d of the fixed collar, or the fixed reference end face 1a and the aforesaid extended end face 5d' respectively in mutually tight contact.
The proposed arrangement prevents the occurrence of friction in the close contact surfaces between the flared hole 2 and the tapered shank 4 due to fretting, corrosion, or the like wear, since the cutting load is received even on the contact surfaces of the extended end face 1a' and the end face 5d of the fixed collar or on the contact surfaces of the fixed reference end face 1a and the extended end face 5d'. However, in the case where the main spindle 1 or tool holder 3 having an extended end face 1a' or 5d' extended by precision processing is used with a tool holder 3 or main spindle 1 which is a conventional product having a tolerance deviation within acceptable tolerances, a problem arises due to the allowed tolerance deviation in their production.
As shown particularly in FIG. 11, if the tool holder 3 of a conventional product has the tapered shank 4 of a slightly shorter length due to a tolerance deviation in production, as illustrated by broken lines, or has the end face 5d of the collar is elongated slightly toward the main spindle due to a tolerance deviation in production, a clearance .beta. occurs between the flared hole 2 and the tapered shank 4 when the aforesaid conventional tool holder 3 is mounted in the particular main spindle 1 having an extended end 1a' created by precision processing, because the end face 5d of the collar of the conventional tool holder 3 contacts the extended end face 1a' of the particular main spindle 1 before the tapered shank 4 tightly engages with the flared hole 2. Therefore, the tool holder 3 cannot be securely mounted in the main spindle 1.
Similarly, as shown in FIG. 12, if the main spindle 1 of a conventional product has the flared hole 2 slightly elongated due to a tolerance deviation in production, as illustrated by broken lines, or the reference end face 1a is slightly elongated forwardly due to a tolerance deviation in production, a clearance .beta. occurs between the flared hole 2 and the tapered shank 4 when the aforesaid conventional main spindle 1 receives the particular tool holder 3 having an extended end face 5d' of the collar extended by precision processing, because the extended end face 5d' of the particular tool holder 3 comes into contact with the reference end face 1a of the conventional main spindle 1 before the tapered shank 4 tightly engages with the flared hole 2. Therefore, the tool holder 3 cannot securely be mounted in the main spindle 1.
In short, it is impossible to use a conventional tool holder or a conventional main spindle with relation to a main spindle or tool holder, respectively, having an opposing extended end face that is extended by a width of the prescribed clearance between the opposing given end faces.